llvm.inc.mk: completely replace detection of GCC includes

This completely removes the hardcoded/os specific path setting for includes.
It directly queries gcc include directories.

It takes what was already done by `makefiles/libc/llvm.inc.mk`.

I replaced the `GCC_MULTI_DIR` handling by givining `CFLAGS_CPU` when
searching for include directories.
`CFLAGS` cannot be used as it will crash when `-target $(CPU_ARCH)` is
added.
It currently requires using deferred variables as `CFLAGS_CPU` can be
overwritten later in the build process.

makefiles/toolchain/llvm.inc.mk

@@ -32,72 +32,41 @@ export SIZE        = $(LLVMPREFIX)size
 # We use GDB for debugging for now, maybe LLDB will be supported in the future.
 include $(RIOTMAKE)/tools/gdb.inc.mk
 
-ifneq (,$(TARGET_ARCH))
-  # Clang on Linux uses GCC's C++ headers and libstdc++ (installed with GCC)
-  # Ubuntu and Debian use /etc/alternatives/gcc-$(TARGET_ARCH)-include/c++/$(GCC_VERSION)
-  # Arch uses /usr/$(TARGET_ARCH)/include/c++/$(GCC_VERSION)
-  # Gentoo uses /usr/lib/gcc/$(TARGET_ARCH)/$(GCC_VERSION)/include/g++-v5
-  GCC_CXX_INCLUDE_PATTERNS ?= \
-  /etc/alternatives/gcc-$(TARGET_ARCH)-include/c++/*/ \
-  /usr/$(TARGET_ARCH)/include/c++/*/ \
-  /usr/lib/gcc/$(TARGET_ARCH)/*/include/g++-v8 \
-  /usr/lib/gcc/$(TARGET_ARCH)/*/include/g++-v7 \
-  /usr/lib/gcc/$(TARGET_ARCH)/*/include/g++-v6 \
-  /usr/lib/gcc/$(TARGET_ARCH)/*/include/g++-v5 \
-  #
+# Include directories from gcc
+#   $1: language <c|cpp>
+#
+#   `realpath` is used instead of `abspath` to support Mingw32 which has issues
+#   with windows formatted gcc directories
+#
+# CFLAGS_CPU is used to get the correct multilib include header.
+gcc_include_dirs = $(realpath \
+    $(shell $(PREFIX)gcc $(CFLAGS_CPU) -v -x $1 -E /dev/null 2>&1 | \
+        sed \
+        -e '1,/\#include <...> search starts here:/d' \
+        -e '/End of search list./,$$d' \
+        -e 's/^ *//')\
+)
 
-  # Try to find the proper multilib directory using GCC, this may fail if a cross-
-  # GCC is not installed.
-  ifeq ($(GCC_MULTI_DIR),)
-    GCC_MULTI_DIR := $(shell $(PREFIX)gcc $(CFLAGS) -print-multi-directory 2>/dev/null)
+ifneq (,$(TARGET_ARCH))
+  ifeq (,$(CFLAGS_CPU))
+    $(error CFLAGS_CPU must have been defined to use `llvm`.)
   endif
 
   # Tell clang to cross compile
-  export CFLAGS     += -target $(TARGET_ARCH)
-  export CXXFLAGS   += -target $(TARGET_ARCH)
+  CFLAGS     += -target $(TARGET_ARCH)
+  CXXFLAGS   += -target $(TARGET_ARCH)
   # We currently don't use LLVM for linking (see comment above).
-  #export LINKFLAGS  += -target $(TARGET_ARCH)
+  # LINKFLAGS  += -target $(TARGET_ARCH)
 
-  # Use the wildcard Makefile function to search for existing directories matching
-  # the patterns above. We use the -isystem gcc/clang argument to add the include
-  # directories as system include directories, which means they will not be
-  # searched until after all the project specific include directories (-I/path)
-  # We sort the list of found directories and take the last one, it will likely be
-  # the most recent GCC version. This avoids using old headers left over from
-  # previous tool chain installations.
-  GCC_CXX_INCLUDES ?= \
-      $(addprefix \
-          -isystem $(firstword \
-              $(foreach pat, $(GCC_CXX_INCLUDE_PATTERNS), $(lastword $(sort $(wildcard $(pat)))))), \
-          /. /$(TARGET_ARCH)/$(GCC_MULTI_DIR) /backward \
-      )
+  # Clang on Linux uses GCC's C and C++ headers and libstdc++ (installed with GCC)
 
-  # If nothing was found we will try to fall back to searching for a cross-gcc in
-  # the current PATH and use a relative path for the includes
-  ifeq (,$(GCC_CXX_INCLUDES))
-    GCC_CXX_INCLUDES := $(addprefix -isystem ,$(wildcard $(dir $(shell which $(PREFIX)gcc))../$(TARGET_TRIPLE)/include))
-  endif
+  # Extract include directories from GCC
+  GCC_C_INCLUDE_DIRS   = $(call gcc_include_dirs,c)
+  GCC_CXX_INCLUDE_DIRS = $(call gcc_include_dirs,c++)
 
-  # Pass the includes to the C++ compilation rule in Makefile.base
-  export CXXINCLUDES += $(GCC_CXX_INCLUDES)
+  GCC_C_INCLUDES   = $(addprefix -isystem ,$(GCC_C_INCLUDE_DIRS))
+  GCC_CXX_INCLUDES = $(addprefix -isystem ,$(GCC_CXX_INCLUDE_DIRS))
 
-  # Some C headers (e.g. limits.h) are located with the GCC libraries
-  GCC_C_INCLUDE_PATTERNS ?= \
-    /usr/lib/gcc/$(TARGET_TRIPLE)/*/ \
-    #
-
-  GCC_C_INCLUDES ?= \
-      $(addprefix -isystem ,$(wildcard $(addprefix \
-          $(lastword $(sort \
-              $(foreach pat, $(GCC_C_INCLUDE_PATTERNS), $(wildcard $(pat))))), \
-          include include-fixed) \
-      ))
-
-  # If nothing was found we will try to fall back to searching for the libgcc used
-  # by an installed cross-GCC and use its headers.
-  ifeq (,$(GCC_C_INCLUDES))
-    GCC_C_INCLUDES := $(addprefix -isystem ,$(wildcard $(addprefix $(dir $(shell $(PREFIX)gcc -print-libgcc-file-name)), include include-fixed)))
-  endif
-
-  export INCLUDES += $(GCC_C_INCLUDES)
+  INCLUDES    += $(GCC_C_INCLUDES)
+  CXXINCLUDES += $(GCC_CXX_INCLUDES)
 endif